This invention relates to in situ processing of oil shale, and more particularly, to a method of processing in stages to increase efficiency.
In co-pending application Ser. No. 716,538, filed Aug. 23, 1976, by Gordon French entitled "Method for In Situ Recovery of Liquid and Gaseous Products from Oil Shale Deposits," assigned to the same assignee as the present invention, and incorporated herein by reference, there is described a multi-level technique by which a vertical in situ oil shale retort is formed in a subterranean formation containing oil shale by mining in the formation a tunnel or drift at the top and a tunnel or drift at the bottom of the desired vertical extent of the retort. An intermediate room having horizontal dimensions corresponding approximately to the horizontal cross section of the retort being formed is also mined or excavated in the portion of the formation to become the in situ retort through an intermediate access tunnel or drift. A vertical columnar void is then formed in the middle of the volume of formation to become the in situ retort, and surrounding formation within the desired retort volume is explosively expanded by blasting to form a retort comprising a cavity containing a rubble pile comprising a fragmented permeable mass of formation particles containing oil shale. Other techniques can be used for forming an in situ oil shale retort containing a fragmented permeable mass of particles.
Once the retort is thus formed in situ, the fragmented mass of formation particles is ignited at the top of the retort to establish a combustion zone in the retort. A combustion supporting inlet gas comprising oxygen is introduced into the combustion zone to advance the combustion zone downwardly through the fragmented mass in the retort. In the combustion zone oxygen in the gas is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By continued introduction of the combustion supporting gas into the combustion zone, the combustion zone is advanced through the retort. An effluent gas from the combustion zone, comprising combustion gas, gas from mineral carbonate decomposition and the portion of the inlet gas that does not take part in the combustion process, passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid hydrocarbon products.
As used herein, the term "processing gas" is used to indicate gas which serves to advance a processing zone such as a combustion zone, a retorting zone, or both a retorting zone and combustion zone, through the fragmented mass in an in situ oil shale retort, and includes, but is not limited to, an oxygen supplying gas introduced into a retort for advancing a combustion zone and retorting zone through a retort and a hot retorting gas which can be introduced into a retort or generated in a combustion zone in a retort for advancing a retorting zone through a retort.
The liquid hydrocarbon products and gaseous hydrocarbon products are cooled by the cooler oil shale particles in the retort on the advancing side of the retorting zone. The liquid products are collected at the bottom of the retort and withdrawn to the surface. An off gas or flue gas containing combustion gas generated in the combustion zone, gas from decomposition of mineral carbonates, gaseous products produced in the retorting zone, and the portion of the inlet gas that does not take part in the combustion process is also withdrawn from the bottom of the retort.
For maximum efficiency of operation, it is desirable to move large volumes of processing gas into the retort and corresponding large volumes of off gas from the retort with minimum expenditure of energy. The large vertical extent of the retort, which may be in the order of 200 or 300 feet, even though containing a permeable mass of formation particles having a void fraction in the order of 15% or more, nevertheless affords a substantial resistance to gas flow, particularly if the explosive expansion operation has not provided complete fragmentation of the formation within the cavity. As a result, excessive pressure drop can be developed to obtain a desired flow rate of gas, thereby requiring substantial expenditure of energy in moving gas through the retort with a resultant drop in overall economics of operation of the retort and recovery of shale oil.
For a given void fraction in the fragmented mass pressure drop in the retort increases as longer retorts are operated. It is desirable to employ long retorts in an effort to recover shale oil from the maximum extent of the formation containing oil shale. If the pressure difference required to obtain adequate gas flow rate is excessive, the energy required for pumps or blowers can be excessive. Further, high pressures can cause leakage of gas from the retort with consequent loss of efficiency. It is, therefore, desirable to keep the pressure difference across the length of the retort as low as possible for adequate flow and also to keep the maximum retort pressure low.